This is true, but they might think those two games can both do well even if they come out around the same time. I'm sure a lot of people are fans of Bioshock, but could care less about GTA and vice versa. I think people who want both will probably do whatever they can to get them on release day(me), or get one on release and then the other for the holidays.

Last year we got Arkham City, BF3, MW3, Uncharted 3, and Skyrim between October 18th and November 11th. They weren't released by the same parent company, but the point I'm making is they all did extremely well because they were all popular franchises. If any two were released by the same company, it would've been a great season for that company.